When punching sheet metal in punching machines, orifices are punched in the sheet metal. Therefore, the sheet metal is pushed in an orifice of a die by means of a stamp. The contour of the stamp corresponds to the contour of the die. In order to create a clearance, the orifice of the die is circumferentially larger than the dimensions of the stamp by a few decimillimeters. Thereby, in the initial phase of the stamping operation, the sheet metal is pushed into the die so that it deforms and tensile stress occurs at the upper surface of the sheet metal. Further, when viewed in cross-section, the upper region of a punching is cut or sheered by sheering forces between the stamp and the die and, finally, the lower portion breaks out due to the reduced carrying cross-section. The tensile stress initially brought in by the deformation is maintained in the sheet metal so that the whole metal sheet or individual portions of the sheet metal are deformed dependent on the size and the number of the punchings. The outer regions of the metal sheet or of the individual portions of the sheet metal except the regions where the sheet metal is clamped fixedly are upwardly bent by the tensile stress. This leads to inaccuracies during further processing and to increased wear of the tool.
The situation can be improved by specifically grinded dies and by the use of an active stripper forcing the sheet metal downward against the die, however, such possibilities are restricted in the case of elongated hole-like tools where no convexity is possible on the die. Furthermore, grinding in a convex manner is only possible in specific cylindrical grinder machines which render the production elaborate and costly. In addition to that, the use of the die grinded in the convex manner is only possible up to a certain grade of punching.
Furthermore, there is the possibility to machine the sheet metal on specific straightening machines in order to restore an evenness of the sheet metals. In these straightening machines, the bent sheet metal is rendered strainless by repeatedly reshaping the sheet metal by means of an arrangement of multiple rollers. Thereby, the sheet is bent in the opposite direction during the movement across the rollers again and again. By such multiple bending of the sheet metal, stresses are removed therefrom and an even, strain-less sheet metal can be obtained. However, this processing is costly because an additional machine and further process steps are necessary. Moreover, this processing is not possible on sheet metals on which remodeling has already been performed in the punching machine.